CO2 Trapping Mechanisms

Once underground, a variety of mechanisms keep the supercritical CO2 securely
stored:

1Structural/stratigraphic trapping -
this is the most dominant of the trapping mechanisms. Once injected,
the supercritical CO2 can be more buoyant than other liquids
that might be present in the pore space. The CO2 will
therefore percolate up through the porous rocks until it reaches
the top of the formation where it meets (and is trapped by) an impermeable
layer of cap-rock. With a man-made CO2 storage site, the
wells that were drilled for injection through the cap-rock would
be sealed with solid physical plugs made of steel and cement, a method
which is already used extensively by the natural gas storage industry.

2 Residual trapping -
this phase of trapping happens very quickly as the porous rock acts
like a tight, rigid sponge. As the supercritical CO2 is
injected into the formation it displaces fluid as it moves through
the porous rock. As the CO2 continues
to move, fluid again replaces it, but some of the CO2 will
be left behind as disconnected - or residual - droplets in the pore
spaces which are immobile, just like water in a sponge. This is often
how the oil was held for millions of years.

3 Solubility
trapping -
Just as sugar dissolves in tea, CO2 dissolves in other fluids
in its gaseous and supercritical state. This phase in the trapping
process involves the CO2 dissolving into the salt water
(or brine) already present in the porous rock. Just as a bottle of
fizzy water is actually slightly heavier than the same bottle filled
with still water, so this salt water containing CO2 is denser
than the surrounding fluids and so will sink to the bottom of the rock
formation over time, trapping the CO2 even more securely.

4 Mineral
trapping -
the final phase of trapping results from the fact that when CO2 dissolves
in water it forms a weak carbonic acid (like mineral water or orange
juice). Over a long time, however, this weak acid can react with
the minerals in the surrounding rock to form solid carbonate minerals,
much like as shellfish use calcium and carbon from seawater to form
their shells or, on a far grander scale, how the White Cliffs of
Dover were formed. This process can be rapid or very slow (depending
on the chemistry of the rock and water in a specific storage site)
but it effectively binds CO2 to the rock.

These trapping processes take place over many years at different
rates from days to years to thousands of years, but in general, geologically
stored CO2 becomes more securely trapped with time.Demonstrations
of various geological storage of CO2 are
already being carried out in a range of projects of varying scale.
Three industrial-scale projects, which inject a minimum of around
3,000 tonnes a day of CO2, have been under way for several
years.

The next step will be to
increase the scale of storage; this will require better mapping of
sources and sinks, and for storage sites to be selected, operated
and monitored in a standardised way. These challenges, which can
be met, will need to be carried out with the support of regulators,
investors and the public.